1. Field of the Invention
The present invention relates to radar applications. More specifically, the present invention relates to methods and apparatus for a wideband radiating element for radar antenna applications.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art:
Phased array antenna systems include at least one element employed for radiating electromagnetic energy into the atmosphere. During the transmission phase, the electromagnetic energy is delivered from a source to an input mounting block via a coaxial cable. Positioned adjacent to the mounting block is a gap formed between a pair of large conductors connected to the leads of the coaxial cable. As the electromagnetic energy is switched across the gap, an electromagnetic wave is generated. The gap serves as a conduit to support the propagation of the energy wave along the large conductors for radiation to the atmosphere.
In order to maximize radiation efficiency and thus minimize energy reflection, the impedance of the input section, the gap and the conductors must be matched. Failure to satisfy this design criteria results in impedance mismatching over the desired frequency bandwidth. Under these conditions, the radiating element is limited to use in a narrower bandwidth. There is a need in the art to develop a radiating element for use with a wide bandwidth array supported by a fiber optic true-time-delay beamforming network. The array is intended to provide a range resolution of one nanosecond. To match this performance, the radiating elements must have compatible bandwidth characteristics. Unfortunately, radiating element designs known in the art are not capable of operating over such a wide bandwidth in an array environment.
An example of a radiating element of the prior art is the flared notch element. The flared notch element incorporates an input mounting block for connecting a coaxial cable to a pair of large flat conductors. One of the two coaxial conductors is connected to a first of the pair of large flat conductors while the other coaxial conductor is connected to the second of the large flat conductors. Microwaves are generated at the input of a slot line or notch located between the pair of large flat conductors. The slot line is narrow at the entry of the mounting block for the purpose of matching the 50.OMEGA. input impedance to the slot line impedance.
The generation and propagation of the microwaves in the slot line of the flared notch element has been discussed at length in the literature. However at certain frequencies, it is difficult to control the microwave radiation from the slot line. This problem occurs because the pair of large flat conductors and the coaxial mounting block do not form a balanced structure. The shunt capacitance existing between the first large conductor and the outer conductor of the coaxial cable destroys the symmetry of the surface current distribution on the radiating element. This is because the outer conductor of the coaxial cable has a larger surface area and is closer to the large flat conductors than the inner conductor of the coaxial cable. This situation will cause the current to flow on the outside surface of the coaxial cable as a return path thereby preventing the low frequency components from propagating down the slot line or notch.
To provide efficient microwave radiation, it is necessary to maintain control of the current over the bandwidth. In order to maintain control, the flow of current must be restricted to a narrow region. Specifically, the current is hard to control because the impedance of the large flat conductors does not remain fixed over a wide range of frequencies. The impedance of the large flat conductors does not remain fixed over a wide range of frequencies because the outer conductor of the coaxial cable has a larger surface area and is closer to the large flat conductors than the inner conductor of the coaxial cable. Thus, the current flow is unsymmetrical which impedes the propagation of certain frequency components of the microwaves. Since the impedance is difficult to control, matching the impedance between the input and the slot line is very difficult.
Unfortunately, this condition in the flared notch element of the prior art results in increased energy reflection and reduced radiating efficiency since the current flow along each radiating portion of the large flat conductors is not symmetrical. The large flat conductors function adequately only for narrow frequency bandwidths. However, for wider bandwidths, the flared notch element does not function adequately. Under these conditions, the impedance of the slot line varies due to the size of the outer coaxial conductor and the proximity to the large flat conductors. Thus, it is difficult to calculate and control the impedance of the slot line resulting in impedance mismatching over a wide bandwidth.
Finally, in the flared notch element, the low frequency components of the wave will be short-circuited by the shunt path through the large flat conductors of an adjacent radiating element in a radar array. This necessitates that the adjacent radiating elements in an array be separated by a distance which utilizes valuable space. Finally, the shunt paths to adjacent radiating elements in an array make it difficult to accurately predict the input impedance of the feed section. Hence, the difficulty in achieving a wideband match increases.
Thus, there is a need in the art for improvements in radiating elements for radar antenna systems which enable impedance matching along the slot line and energy propagation over a wide frequency bandwidth.